An Egyptian police officer reacts as he walks through the site of a car bomb explosion that struck police headquarters Friday in Cairo. At least six people died in a string of bombings across Cairo over 10 hours.

CAIRO — A truck bomb struck the main security headquarters in Cairo on Friday, one of a string of bombings targeting police within a 10-hour period, killing six people. The most significant attack yet in the Egyptian capital fueled a furious backlash against the Muslim Brotherhood amid rising fears of a militant insurgency.

The mayhem on the eve of the third anniversary of Egypt’s once-hopeful revolution pointed to the dangerous slide Egypt has taken since last summer’s military ouster of Islamist President Mohammed Morsi: A mounting confrontation between the military-backed government and Islamist opponents amid the escalating violence.

In the hours after the blast, angry residents — some chanting for the “execution” of Brotherhood members — joined police in clashes with the group’s supporters holding their daily street protests against the government. Smoke rose over Cairo from fires, and fighting across the country left 14 more people dead.

Saturday, the anniversary of the start of the 18-day uprising that ousted autocrat Hosni Mubarak, raised the potential for new violence, as both military supporters and the Islamists vowed to take to the streets with rival rallies.

After the blasts, interim President Adli Mansour vowed to “uproot terrorism.” The state “will not show them pity or mercy,” he said. “We … will not hesitate to take the necessary measures.”

That could spell an escalation in the crackdown that the government has waged against Morsi’s Muslim Brotherhood since his July 3 ouster.

Thousands of Islamists have already been arrested and hundreds killed, with authorities accusing the group of being behind militant violence. The Brotherhood, which allied with some radical groups while in power, denies the claim, saying the government is using it to justify its drive to eliminate it as a rival.

The day’s violence began with the 6:30 a.m. blast at the security headquarters on downtown Bab el-Khalq Square. The blast ripped down a main avenue, knocking out shop windows for more than 500 yards. The eight-story headquarters’ facade was shattered and a 6-foot crater was blasted into the pavement.

The explosion also wrecked Cairo’s renowned Islamic Arts Museum, directly across the street, blasting out its windows, causing ceilings to collapse, smashing display cases of porcelain and glasswork and breaking water pipes that sprayed over manuscripts.

About two hours later, attackers threw a bomb at a police car near a metro station in the Dokki district on the other side of the Nile River. A third, smaller blast targeted the Talbiya police station about two miles from the famed Giza Pyramids.

As police drove back from clashes with Brotherhood supporters in the Giza district, they were hit by a roadside explosive that killed one person and wounded four others.

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